There are many applications in optical communication systems for a high power, low noise broadband light source. For example, efforts are now being made toward “spectral slicing”, where a common light source is used to generate a large number of (independent) wavelength division multiplexed (WDM) optical signals. Using spectral slicing, therefore, a single light source may be employed to take the place of a multiple number of separate, narrow linewidth lasers, as was required in the prior art. Other applications for a continuum light source include, but are not limited to, frequency metrology, device characterization, dispersion measurements made on specialty fibers, and the determination of optical grating transmission characteristics. All of these various diagnostic tools may be greatly enhanced by the availability of such a broadband source.
In general, continuum generation involves the launching of relatively high power laser radiation (in most cases, pulsed radiation) into an optical material where the pulse train undergoes significant spectral broadening as a result of the nonlinearity of the material. Most prior art arrangements for providing continuum generation involve the use of highly nonlinear optical fibers, microstructured fibers and/or nonlinear planar waveguides. In each arrangement, a guiding structure is defined and used to confine the light as it passes through the nonlinear material.
Continuum light of wavelengths spanning more than one octave have been generated in microstructured and tapered optical fibers by launching light pulses having durations on the order of femtoseconds (10−15 seconds) into the endface of the fibers. The extreme spectra thus produced are useful, for example, in measuring and stabilizing pulse-to-pulse carrier envelope phase, as well as in high-precision optical frequency combs.
It is also well known that bulk optic materials (i.e., photonic crystals) can be fabricated with periodic modulations of their refractive index. Examples include photonic bandgap (PBG) structures, in which a microstructure is patterned into a bulk optic material such that two (or more) distinct refractive indexes (e.g., air and silica) yield a periodic or quasi-periodic pattern in two or three dimensions within the waveguiding layer. In such structures, therefore, a “bandgap” results where one or more frequencies of an applied optical signal will not propagate through the bulk material. A continuum of light may also be generated by launching high power pulses through bulk materials that do not contain any type of guiding refractive index structure. Similar to guided wave continua, the chromatic dispersion of the bulk material plays a significant role in determining the continuum generation properties. One problem with such structures, however, is that there is little control over the material dispersion and, as a result, little control/flexibility in the generated continuum. Thus, a means to control the dispersion of a bulk, non-guiding material would be advantageous.